Memory system

ABSTRACT

According to one embodiment, a memory system includes a semiconductor memory and a controller. The memory system is capable of executing a first operation and a second operation. In the first operation, the controller issues a first command sequence, the semiconductor memory applies a first voltage to a first word line and applies a second voltage to a second word line to read data from the first memory, and the read data is transmitted to the controller from the semiconductor memory. In the second operation, the controller issues a second command sequence, the semiconductor memory applies a third voltage to the first word line and applies a fourth voltage to the second word line, and data held in the memory cell array is left untransmitted to the controller.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims benefit under 35 U.S.C.§ 120 to U.S application Ser. No. 17/370,535, filed Jul. 8, 2021, whichis a continuation of and claims benefit under 35 U.S.C. 120 to U.Sapplication Ser. No. 16/826,595, filed Mar. 23, 2020, now U.S. Pat. No.11,086,573, issued Aug. 10, 2021, which is a continuation of and claimsbenefit under 35 U.S.C. § 120 to U.S application Ser. No. 16/118,543,filed Aug. 31, 2018, now U.S. Pat. No. 10,635,354, issued Apr. 28, 2020,which is based upon and claims the benefit of priority under 35 U.S.C.119 from Japanese Patent Application No. 2018-052646, filed Mar. 20,2018, the entire contents of each of which are incorporated herein byreference.

FIELD

Embodiments described herein relate generally to a memory system.

BACKGROUND

A semiconductor memory in which memory cells are three-dimensionallyarranged is known.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a memory system according to the firstembodiment;

FIGS. 2 and 3 respectively are a circuit diagram and a sectional view ofa block according to the first embodiment;

FIG. 4 is a graph showing threshold distribution of memory cellsaccording to the first embodiment;

FIGS. 5 and 6 respectively are conceptual diagrams of a shift table anda history table according to the first embodiment;

FIG. 7 is a conceptual diagram of a refresh read flag and a block copyflag according to the first embodiment;

FIG. 8 is a flowchart of the read operation according to the firstembodiment;

FIG. 9 is a command sequence of set feature according to the firstembodiment;

FIGS. 10 and 11 are conceptual diagrams she information in a register inthe NAND flash memory according to the first embodiment;

FIG. 12 is a flowchart of the refresh read operation according to thefirst embodiment;

FIG. 13 is a timing chart of various signals in the read operation andthe refresh read operation according to the first embodiment;

FIGS. 14 and 15 respectively are circuit diagrams of a block in a normalread and a refresh read according to the first embodiment;

FIG. 16 is a flowchart of a read operation and a refresh read operationaccording to the first embodiment;

FIG. 17 is a flowchart of a read operation and a block copy operationaccording to the first embodiment;

FIG. 19 shows threshold distributions of memory cells according to thefirst embodiment;

FIG. 19 is a flowchart of the refresh read operation according to thesecond embodiment;

FIG. 20 is a flowchart of a read operation and a refresh read operationaccording to the second embodiment;

FIG. 21 is a flowchart of the read operation and the refresh readoperation according to the third embodiment;

FIGS. 22, 23, and 24 are circuit diagrams of a block in the refresh readaccording to the first modification of the first to third embodiments;

FIGS. 25 and 26 are conceptual diagrams of the history table accordingto the second modification of the first to third embodiments; and

FIGS. 27, 28, 29, and 30 are timing charts of various signals in theread operation and the refresh read operation according to the thirdmodification of the first to third embodiments.

DETAILED DESCRIPTION

In general, according to one embodiment, a memory system includes asemiconductor memory and a controller. The memory system is capable ofexecuting a first operation and a second operation after the firstoperation. The semiconductor memory includes a memory cell arrayincluding a first memory cell and a second memory cell configured tohold data and coupled respectively to a first word line and a secondword line. The controller is configured to control the semiconductormemory. In the first operation, the controller issues a first commandsequence and, in response to the first command sequence, thesemiconductor memory applies a first voltage to the first word line andapplies a second voltage to the second word line to read data from thefirst memory cell, and the read data is transmitted from thesemiconductor memory to the controller. In the second operation, thecontroller issues a second command sequence and, in response to thesecond command sequence, the semiconductor memory applies a thirdvoltage to the first word line and applies a fourth voltage to thesecond word line, and data held. in the memory cell array is leftuntransmitted to the controller. The second operation is executed afterthe first operation without intervention of a write operation and eraseoperation on the first memory cell. The third voltage turns on the firstmemory cell regardless of data held in the first memory cell. The secondvoltage and the fourth voltage turn on the second memory cell regardlessof data held in the second memory cell.

1. First Embodiment

A memory system according to the first embodiment will be described. Thefollowing description will be provided while using, as an example, amemory system including a NAND flash memory as a semiconductor memorydevice.

1.1 Configuration

1.1.1 Overall Configuration of Memory System

A rough overall configuration of a memory system according to thepresent embodiment will be described with reference to FIG. 1 .

As shown in FIG. 1 , a memory system 1 includes a NAND flash memory 100and a controller 200. The NAND flash memory 100 and the controller 200may form one semiconductor device in combination, for example. Thesemiconductor device is, for example, a memory card such as an SD™ card,or a solid state drive (SSD).

a The NAND flash memory 100 includes a plurality of memory cells tonon-volatilely store data. The controller 200 is coupled to the NANDflash memory 100 by a NAND bus, and is coupled to a host apparatus 300by a host bus. The controller 200 controls the NAND flash memory 100,and accesses the NAND flash memory 100 in response to an instructionreceived from the host apparatus 300. The host apparatus 300 is, forexample, a digital camera or a personal computer, and the host bus is,for example, a bus compliant with an SD™ interface. The NAND busperforms signal transmission/reception compliant with a NAND interface.

1.1.2 Configuration of Controller 200

Details of the configuration of the controller 200 will be describedwith continuous reference to FIG. 1 . As shown in FIG. 1 , thecontroller 200 includes a host interface circuit 210, an embedded memory(RAM) 220, a processor (CPU) 230, a buffer memory 240, a NAND interfacecircuit 250, and an error checking and correcting (ECC) circuit 260.

The host interface circuit 210 is coupled to the host apparatus 300 viathe host bus to transfer instructions and data received from the hostapparatus 300 respectively to the processor 230 and the buffer memory240. The host interface circuit 210 also transfers data in the buffermemory 240 to the host apparatus 300 in response to an instruction fromthe processor 230.

The processor 230 controls the operation of the entire controller 200.For example, upon receipt of a read instruction from the host apparatus300, the processor 230 issues, in response thereto, a read command tothe NAND interface circuit 250 similar process is performed when writingand erasing. The processor 230 also executes various processes, such aswear leveling, for managing the NAND flash memory 100.

The NAND interface circuit 250 is coupled to the NAND flash memory 100via the NAND bus to communicate with the NAND flash memory 100. Based oninstructions received from the processor 230, the NAND interface circuit250 transmits various signals to, and receives various signals from, theNAND flash memory 100.

The buffer memory 240 temporarily holds write data and read data.

The embedded memory 220 is, for example, a semiconductor memory, such asa DRAM or an SRAM, and is used as a work area of the processor 230. Theembedded memory 220 holds firmware for managing the NAND flash memory100, and various management tables and the like, such as a shift table,a history table, and a flag table to be described later.

The ECC circuit 260 performs error detection and error correctionprocesses on data stored in the NAND flash memory 100. Namely, the ETCcircuit 260 generates an error correction code and provides write datawith the error correction code in data writing, and decodes the errorcorrection code in data reading.

1.1.3 Configuration of NAND Flash Memory 100

Next, a configuration of the NAND flash memory 100 will be described. Asshown in FIG. 1 , the NAND flash memory 100 includes a memory cell array110, a row decoder 120, a driver circuit 130, a sense amplifier 140, anaddress register 150, a command register 160, and a sequencer 170.

The memory cell array 110 includes a plurality of blocks BLK eachincluding a plurality of nonvolatile memory cells associated with rowsand columns. FIG. 1 shows four blocks BLK0 to BLK3, as an example. Thememory cell array 110 stores data provided from the controller 200.

The row decoder 120 selects one of the blocks BLK0 to BLK3 based on ablock address BA in the address register 150, and further selects a rowin the selected block BLK.

The driver circuit 130 supplies a voltage to the selected block BLK viathe row decoder 120 based on a page address PA in the address register150.

In data reading, the sense amplifier 140 senses data read from thememory cell array 110, and performs a necessary arithmetic operation.Then, the sense amplifier 140 outputs the data DAT to the controller200. In data writing, the sense amplifier 140 transfers write data DATreceived from the controller 200 to the memory cell array 110.

The address register 150 holds an address ADD received from thecontroller 200. The address ADD includes the above-mentioned blockaddress BA and page address PA. The command register 160 holds a commandCMD received from the controller 200.

The sequencer 170 controls the operation of the entire NAND flash memory100 based on the command CMD held in the command register 160.

Next, a configuration of the block BLK will be described with referenceto FIG. 2 . As shown in FIG. 2 , the block BLK includes, for example,four string units SU (SU0 to SU3). Each string unit SU includes aplurality of NAND strings 10.

Each NAND string 10 includes, for example, eight memory cell transistorsMT (MT0 to MT7) , two dummy transistors DT (DTD and DTS), and twoselection transistors ST1 and ST2. Each memory cell transistor MTincludes a control gate and a charge accumulation layer, andnon-volatilely holds data. The memory cell transistors MT are coupled inseries between the source of selection transistor ST1 and the drain ofselection transistor ST2 via dummy transistors DTD and DTS,respectively.

Like the memory cell transistor MT, each dummy transistor DT includes acontrol gate and a charge accumulation layer. However, unlike the memorycell transistor MT, the dummy transistor DT is not used for holdingdata, but functions as a mere current path in the NAND string 10.Namely, the threshold of the dummy transistor DT is set at a low valueso that the dummy transistor DT is always turned on in read operationsand write operations. The threshold of the dummy transistor DT may beset at a predetermined value by performing a write operation on thedummy transistor DT to control the amount of charge in the chargeaccumulation layer.

The gates of selection transistors ST1 in string units SU0 to SU3 arecoupled to respective selection gate lines SGD0 to SGD3. On the otherhand, the gates of selection transistors ST2 in string units SU0 to SU3are coupled in common to, for example, selection gate line SGS. Thegates of selection transistors STS in string units SU0 to SU3 may ofcourse be coupled to respective different selection gate lines SGS0 toSGS3. The control gates of the memory cell transistors MT0 to MT7 in thesame block BLK are coupled in common to respective word lines WL0 toWL7, and the control gates of the dummy transistors DTD and DTS in thesame block BLK are coupled in common to respective dummy word lines WLDDand WLDS.

The drains of selection transistors ST1 of the NAND strings 10 in thesame column in the memory cell array 110 are coupled in common to a bitline BL (BL0 to BL(L-1), where (L-1) is a natural number equal to orlarger than 2). Namely, the NAND strings 10 in the same column of aplurality of blocks BLK are coupled in common to a bit line BL.Moreover, the sources of a plurality of selection transistors ST2 arecoupled in common to a source line SL.

Namely, the string unit SU includes a plurality of NAND strings 10coupled to different bit lines BL and coupled to the same selection gateline SGD. The block BLK includes a plurality of string units SU sharingword lines WL. The memory cell array 110 includes a plurality of blocksBLK sharing bit lines BL.

FIG. 3 is a sectional view of a partial region of the block BLK. Asshown in FIG. 3 , a plurality of NAND strings 10 are formed on a p-typewell region 20. Namely, three interconnect layers 27 functioning, forexample, as selection gate line SGS, ten interconnect layers 23functioning as dummy word line WLDS, word lines WL0 to WL7, and dummyword line WLDD, and three interconnect layers 25 functioning, forexample, as selection gate line SGD, are sequentially stacked above thewell region 20. Insulating films (not shown) are formed between thestacked interconnect layers.

A pillar-shaped conductor 31 extending through interconnect layers 25,23, and 27 to reach the well region 20 is formed. A gate insulating film30, a charge accumulation layer (insulating film) 29, and a blockinsulating film 28 are sequentially formed on the side surface of theconductor 31, thereby forming memory cell transistors MT, dummytransistors DT, and selection transistors ST1 and ST2. The conductor 31functions as a current path of the NAND string 10, and is used as aregion in which a channel of each transistor is formed. The upper end ofthe conductor 31 is coupled to a metal interconnect layer 32 thatfunctions as a bit line BL.

In a surface region of the well region 20, an n⁺-type impurity diffusionlayer 33 is formed. A contact plug 35 is formed on the diffusion layer33, and is coupled to a metal interconnect layer 36 that functions as asource line SL. In the surface region of the well region 20, a p⁺-typeimpurity diffusion layer 34 is also formed. A contact plug 37 is formedon the diffusion layer 34, and is coupled to a metal interconnect layer38 that functions as a well interconnect CPWELL. The well interconnectCPWELL is used to provide a potential to the conductor 31 via the wellregion 20.

A plurality of configurations as described above are arranged in thedepth direction of the sheet of FIG. 3 , and a set of a plurality ofNAND strings 10 aligned in the depth direction form a string unit SU.

In the present embodiment, one memory cell transistor MT can hold, forexample, 3-bit data. The bits of the 3-bit data will be referred to as alower bit, a middle bit, and an upper bit in ascending order from theleast significant bit. A set of lower bits held in memory cells coupledto the same word line will be referred to as a lower page, a set ofmiddle bits will be referred to as a middle page, and a set of upperbits will be referred to as an upper page. Namely, three pages areassigned to one word line WL, and the block BLK including eight wordlines WL has a capacity of 24 pages. In other words, “page” may also bedefined as a part of a memory space formed by memory cells coupled tothe same word line. Data writing and data reading may be performed inunits of pages.

FIG. 4 is a diagram showing storable data, threshold distribution, andvoltages used in reading of memory cell transistors MT.

As shown in FIG. 4 , the memory cell transistors MT may take eightstates in accordance with their threshold voltages. The eight stateswill be referred to as an “Er” state, “A” state, “B” state, “C” state, .. . , and “G” state in ascending order of threshold voltage.

The threshold voltages of memory cell transistors MT in the “Er” stateare lower than voltage VA, and the “Er” state corresponds to adata-erased state. The threshold voltages of memory cell transistors MTin the “A” state are equal to or higher than voltage VA, and lower thanvoltage VT (>VA). The threshold voltages of memory cell transistors MTin the “B” state are equal to or higher than voltage VB, and lower thanvoltage VC (>VB). The threshold voltages of memory cell transistors MTin the “C” state are equal to or higher than voltage VC and lower thanvoltage VD (>VC). The threshold voltages of memory cell transistors MTin the “D” state are equal to or higher than voltage VD and lower thanvoltage VE (>VD). The threshold voltages of memory cell transistors MTin the “E” state are equal to or higher than voltage VE and lower thanvoltage VF (>VE). The threshold voltages of memory cell transistors MTin the “F” state are equal to or higher than voltage VT and lower thanvoltage VG (>VF). The threshold voltages of the memory cell transistorsMT in the “G” state are equal to or higher than voltage VG and lowerthan voltage VREAD. Of the eight states accordingly distributed, the “G”state is the highest threshold voltage state. Note that voltage VREAD isa voltage applied to non-selected word lines in read operations, andturns on memory cell transistors MT regardless of held data.

The above-described threshold distribution is obtained by writing 3-bit(3-page) data constituted by the above-mentioned lower bit, middle bit,and upper bit. The relationship between the above eight states and thelower bit, middle bit, and upper bit is as follows:

-   “Er” state: “111” (in the order of “upper/middle/lower”)-   “A” state: “110”-   “B” state: “100”-   “C” state: “000”-   “D” state: “010”-   “E” state: “011”-   “F” state: “001”-   “G” state: “101”

Only one of the three bits is different between data corresponding toadjacent two states is the threshold distribution.

Accordingly, when the lower bit is read, a voltage corresponding to theboundary where the value (“0” or “1”) of the lower bit changes may beused; this also applies when reading the middle bit and the upper bit.

Namely, as shown in FIG. 4 , is lower page reading, voltage VA, whichdistinguishes between the “Er” state and the “A” state, and voltage VE,which distinguishes between the “D” state and the “E” state, are used asread voltages. The read operations using voltages VA and VT will bereferred to as read operations AR and ER, respectively.

In middle page reading, voltage VB, which distinguishes between the “A”state and the “B” state, voltage VD, which distinguishes between the “C”state and the “D” state, and voltage VF, which distinguishes between the“E” state and the “F” state, are used as read voltages. The readoperations using voltages VB, VD, and VF will be referred to as readoperations BR, DR, and FR, respectively.

In upper page reading, voltage VC, which distinguishes between the “B”state and the “C” state, and voltage VG, which distinguishes between the“F” state and the “G” state, are used as read voltages. The readoperations using voltages VC and VG will be referred to as readoperations CR and GR, respectively.

Data erasing can be performed in units of blocks BLK, or smaller units.An erase method is described in, for example, U.S. patent applicationSer. No. 13/235,389 filed on Sep. 18, 2011, titled “NONVOLATILESEMICONDUCTOR MEMORY DEVICE.” An erase method is also described in U.S.patent application Ser. No. 12/694,690 filed on Jan. 27, 2010, titled“NON-VOLATILE SEMICONDUCTOR STORAGE DEVICE.” Furthermore, an erasemethod is described in U.S. patent application Ser. No. 13/483,610 filedon May 30, 2012, titled “NONVOLATILE SEMICONDUCTOR MEMORY DEVICE ANDDATA ERASE METHOD THEREOF.” The entire contents of those patentapplications are incorporated herein by reference.

The memory cell array 110 may have other configurations. A configurationof the memory cell array 110 is described in, for example, U.S. patentapplication Ser. No. 12/407,403 filed on Mar. 19, 2009, titled “THREEDIMENSIONAL STACKED NONVOLATILE SEMICONDUCTOR MEMORY.” A configurationof the memory cell array 110 is also described in U.S. patentapplication Ser. No. 12/406,524 filed on Mar. 18, 2009, titled “THREEDIMENSIONAL STACKED NONVOLATILE SEMICONDUCTOR. MEMORY,” U.S. patentapplication Ser. No. 12/679,991 filed on Mar. 25, 2010, titled“NON-VOLATILE SEMICONDUCTOR STORAGE DEVICE AND METHOD OF MANUFACTURINGTHE SAME,” U.S. patent application Ser. No. 12/532,030 filed on Mar. 23,2009, titled “SEMICONDUCTOR MEMORY AND METHOD FOR MANUFACTURING SAME.”The entire contents of those patent applications are incorporated hereinby reference.

1.1.4 Shift Table

Next, a shift table will be described. As mentioned above, thecontroller 200 holds a shift table in, for example, the memory 220. Aconcept of the shift table will be described with reference to FIG. 5 .FIG. 5 is a conceptual diagram of an example of the shift table.

As shown in FIG. 5 , the shift table includes a plurality of (twelve inthe example of FIG. 5 ) entries. The entries are sequentially assignedwith indexes “1” to “12.” Each entry holds information indicating ashift amount of the read voltage applied to a selected word line WL ineach of the read operations AR, BR, CR, . . . , and GR.

The shift amount will be described below. The threshold distributionscorresponding to the “Er” state, “A” state, . . . , and “G” statedescribed in relation to FIG. 4 are apart from one another. Accordingly,the states can be distinguished from one another by voltages VA, VB, . .. , and VG. However, threshold distributions may be shifted to the lowvoltage side by interference with an adjacent cell (which will bereferred to as an inter-cell interference effect) after time has elapsedfrom data writing. The shift amount increases as the elapsed timeincreases and as the number of writes in the block increases.Accordingly, adjacent threshold distributions may overlap each other byincrease of the distribution width of the threshold distribution of eachstate.

If the threshold distributions change due to the inter-cell interferenceeffect as described above, data may not be correctly read by theabove-mentioned voltages VA, VB, . . . , and VG. Specifically, when thenumber of error bits included in read data exceeds the number of bitsthat can be corrected by the ECC circuit 260, a data read results infailure.

In such a case, the controller 200 shifts the read voltages fromvoltages VA, VB, . . . , and VG (which are referred to as defaultvoltages), and retry a data read. This is called a shift read.Information indicating shift amounts ΔV from default voltages used forthe shift read is held in the shift table shown in FIG. 5 .

The shift table of this example indicates shift amounts ΔVAj, ΔVBj, . .. , and ≢VGj (where j is an index, i.e., one of natural numbers 1 to 12;which will be collectively referred to as shift amounts ΔV when notdistinguished from one another) for respective read operations AR, BR, .. . , and GR. The shift amounts ΔVAj, ΔVBj, . . . , and ΔVGjcorresponding to an index j may be all different values, partly thesame, or all the same. Those shift amounts ΔV are set at appropriatevalues as suited by, for example, the controller 200.

According to the shift table of this example, when the influence of theinter-cell interference effect is small, the applied index value issmall. Namely, when the influence of the inter-cell interference effectis large, the applied index value is large. Namely, the shift amounts ΔVcorresponding to index=1 (i.e., ΔVA1, ΔVB1, . . . , and ΔVG1) areoptimized for the case where the influence of the inter-cellinterference effect is the smallest. In contrast, the shift amounts ΔVcorresponding to index=12 (i.e., ΔVA12, ΔVB12, . . . , and ΔVG12)corresponding to index=12 are optimized for the case where the influenceof the inter-cell interference effect is the largest.

The degree of the influence of the inter-cell interference effect mayvary depending on which of the “Er” to “G” states the memory celltransistor MT is in. For example, in the case of the inter-cellinterference effect where threshold distributions are shifted to the lowvoltage side as time elapses, which is a focus of the presentembodiment, a higher threshold voltage state receives a largerinfluence. Namely, the inter-cell interference effect has a largeinfluence on read operations GR and FR. On the other hand, the influenceon read operations CR and DR does not tend to be very large.Accordingly, for example, the following relationship may hold:

-   -   |ΔVG12|>|ΔVG1|    -   |ΔVG12|×|VC12| and |ΔVD12|        This relationship is merely an example, and the present        embodiment is not limited to such a case. The reason for using        absolute values is that the shift amounts ΔV take negative        values when the influence of the inter-cell interference effect        is a threshold change to the negative voltage side. Each shift        amount ΔV may of course have a negative value or a positive        value, as long as the value is appropriate for a shift read.

The controller 200 also stores one of the indexes as a first referencevalue, and another one of the indexes as a second reference value. Theindex determined as the second reference value corresponds to shiftamounts for a larger influence of the inter-cell interference effectthan the index determined as the first reference value. In the exampleof FIG. 5 , index=7 is the first reference value, and index=10 is thesecond reference value. The first reference value is used as a criterionto judge whether to perform a refresh read, and the second referencevalue is used as a criterion to judge whether to perform a block copyoperation. The refresh read and block copy will be described later indetail.

The shift table configured as described above and information on whichindexes are the first reference value and the second reference value areheld in, for example, one block BLK of the NAND flash memory 100. Theyare read by the controller 200, for example, immediately after the poweris turned on, and held in, for example, the memory 220.

1.1.5 History Table

The controller 200 further holds a history table. The history table willbe described with reference to FIG. 6 . FIG. 6 is a conceptual diagramof an example of the history table.

As shown in FIG. 6 , the history table holds a relationship between wordlines WL0 to WL7 in each logical block LB, and indexes in the shifttable applied to those word lines WL0 to WL7. Hereinafter, each blockBLK included in the memory cell array 110 of the NAND flash memory 100may be referred to as a physical block for distinction from the logicalblock LB. Note, however, that when a block is merely called “block BLK”herein, the “block BLK” means the physical block.

A unique address is assigned to each physical block BLK. This is calleda physical address. In contrast, the host apparatus 300 accesses thecontroller 200 using a logical address which is different from thephysical address. Physical blocks BLK designated by the logical addressare a logical block LB which is, for example, a set of a plurality ofphysical blocks BLK. The relationship between the logical block LB andthe physical blocks BLK may always change. For example, when logicalblock LB0 corresponds to physical blocks BLK0 to BLK3 at a certain pointin time, logical block LB0 corresponds to other physical blocks BLK1 toBLK7 at a later point. Therefore, for example, the buffer memory 240 ofthe controller 200 holds the relationship between a logical address andphysical addresses as a logical/physical address conversion table.

The history table holds information on shift amounts ΔV of read voltagesto be applied to each word line WL of the logical block LB by use of,for example, an index. The controller 200 determines read voltages to beapplied to the selected word line WL with reference to the historytable.

For example, in the example of FIG. 6 , index=1 is assigned to wordlines WL0 of logical block LB0. This means that when read voltages areapplied to word lines WL0 of physical blocks BLK corresponding tological block LB0, the shift amounts (ΔVA1, ΔVB1, . . . , and ΔVG1)corresponding to index=1 in the shift table shown in FIG. 5 are appliedto the default values.

The shift amounts designated by the history table are set in the NANDflash memory 100 by the controller 200 by use of, for example, a setfeature command. Therefore, the shift amounts ΔV need not be designatedfor every read operation. However, when indexes are updated in thehistory table, the controller 200 resets shift amounts for the NANDflash memory 100. The set feature command is a command that can changevarious settings, such as a voltage and filming, etc. in the NAND flashmemory 100.

The history table configured as described above is held in, for example,one block BLK of the NAND flash memory 100. Then, the history table isread by the controller 200, for example, immediately after the power ofthe memory system 1 is turned on, and held in, for example, the memory220. When the power of the memory system 1 is shut off, the historytable held in, for example, the memory 220 is written one block BLK ofthe NAND flash memory 100. Namely, the history table in the NAND flashmemory 100 is updated.

1.1.6 Flag Table

The controller 200 further holds a flag table. The flag table will bedescribed with reference to FIG. 7 . FIG. 7 is a conceptual diagram ofan example of the flag table.

As shown in FIG. 7 , the flag table holds a refresh read flag and ablock copy flag for each logical block LB. “1” is set as the refreshread flag and the block copy flag when a refresh read and a block copyare respectively reserved by the controller 200, for example.

The refresh read is an operation for reducing the influence of theinter-cell interference effect described in item 1.1.4. Morespecifically, for example, an operation similar to the read operation isperformed, and a voltage is applied to a word line WL. A read disturb isthereby caused in the memory cell transistor MT, and thresholddistributions influenced by the inter-cell interference effect areshifted to the high voltage side. Namely, an attempt to move thethreshold distributions closer to ideal states is made. This is therefresh read. In the present embodiment, this operation is called a“refresh read”; however, no data is read from the NAND flash memory 100to the controller 200. In the refresh read, read data is not transferredfrom the NAND flash memory 100 to the controller 200.

The block copy is an operation performed when data is difficult to becorrectly read by the refresh read or any other means, so as to copyeffective data in all the physical blocks BLK corresponding to thelogical block LB to other physical blocks BLK. Data in the copy-sourcephysical blocks BLK are all erased.

For each logical block LB, the controller 200 reserves a refresh readwhen the applied index in the history table exceeds the first referencevalue (“7” in the example of FIG. 5 ), and reserves a block copy whenthe index exceeds the second reference value (“10” in the example ofFIG. 5 ).

The flag table configured as described above is held in, for example,one block BLK of the NAND flash memory 100. Then, like the historytable, the flag table is read by the controller 200, for example,immediately after the power of the memory system 1 is turned on, andheld in, for example, the memory 220. When the power of the memorysystem 1 is shut off, the flag table held in, for example, the memory220 is written in one block BLK of the NAND flash memory 100.

1.2 Read Operation and Refresh Read Operation

Next, a data read operation and refresh read operation according to thepresent embodiment will be described.

1.2.1 Read Operation

A flow of the read operation by the controller 200 will be describedwith reference to FIG. 8 . FIG. 8 is a flowchart of the read operation.The operation of the controller 200 FIG. 8 is executed by control of theprocessor 230, for example.

The controller 200 first refers to the shift table and the history tablein the memory 220 (step S10). The controller 200 thereby ascertainsshift amounts for each word line WL of each physical block BLK. At acertain time, the controller 200 issues a set feature command, and setsthe ascertained shift amounts in the NAND flash memory 100. This processis shown in FIGS. 9 and 10 . FIG. 9 is a command sequence of setfeature, and FIG. 10 shows an example of information set in the NANDflash memory 100 by the set feature.

Hereinafter, attention is focused on logical block LB0, and the casewhere logical block LB0 corresponds to four physical blocks BLK0 to BLK3will be described as an example. The controller 200 recognizes thatlogical block LB0 corresponds to physical blocks BLK0 to BLK3 based onthe logical/physical address conversion table in the memory 220, forexample. The controller 200 also ascertains indexes applied to logicalblock LB0 based on the history table, and recognizes shift amounts to beused in logical block LB0 based on the indexes and the history table.

Then, the controller 200 sets in the NAND flash memory 100 shift amountsto be applied to the physical blocks BLK0 to BLK3 corresponding tological block LB0, in response to a set feature command. Namely, asshown in FIG. 9 , the controller 200 first transmits command “EFh” tothe NAND flash memory 100. Command “EFh” is a command that announces asetting change to the NAND flash memory 100. Subsequently, thecontroller 200 transmits address “ZZh.” Address “ZZh” is an address thatdesignates a register that holds setting values to be changed by the setfeature command in the NAND flash memory 100. After that, the controller200 transmits data over four cycles (data “D0” to “D3”). The data “D0”to “D3” includes information on the shift amounts to be applied tophysical blocks BLK0 to BLK3.

As a result, information as shown in FIG. 10 is held in the designatedregister in the NAND flash memory 100. Namely, the register holds shiftamounts ΔV used to perform read operations AR, BR, . . . , and GR onword lines WL0 to WL7 of physical blocks BLK0 to BLK3. The driver 130 ofthe NAND flash memory 100 applies, as read voltages VCGRV, voltagesobtained by adding the shift amounts held in the register to the defaultvoltages VA, VB, . . . , and VG for reading to a selected word line WLvia the row decoder 120. Namely, voltages (VA+ΔVA), (VB+ΔVB), . . . ,and (VG+ΔVG) become default values of read voltages VCGRV instead oforiginal voltages VA, VB, . . . , and VG.

When shift amounts are set directly according to the history table readfrom the NAND flash memory 100, the set feature command need notnecessarily be used. For example, when the power is turned on, the NANDflash memory 100 itself (for example, the sequencer 170) may set theshift amounts shown in FIG. 10 in the register based on the historytable and shift table read from the memory cell array 110.

Upon receipt of a data request instruction from the host apparatus 300,the controller 200 issues a normal read command (step S11). The normalread command does not include information on shift amounts of readvoltages. Therefore, the NAND flash memory 100 reads data by usingdefault read voltages, and transmits the data to the controller 200.Namely, when, for example, word line WL2 of block BLK0 is selected,voltages (VA+ΔVA7), (VB+ΔVB7), (VC+ΔVC7), . . . , and (VG+ΔVG7) are usedas read voltages VCGRV.

In the controller 200 that has received read data from the NAND flashmemory 100, the ECC circuit 260 performs error detection. When the readdata does not include an error, or an error can be corrected (hard bitdecoding) by the FCC circuit 260 (YES in step S12), the data readoperation is completed.

When the number of error bits included in the read data exceeds thenumber of bits that can be corrected by the ECC circuit 260 (NO in stepS12), and the history table designates the last index (“12” in theexample of FIG. 5 ) (YES in step S13), the controller 200 reserves acopy operation of a logical block (step S14). Namely, in the flag table,for example, in the memory 220, a block copy flag is set for the logicalblock LB corresponding to the physical blocks BLK to be read. After stepS14, the controller 200 executes some operation for correctly readingdata (step S15). The operation is, for example, a Vth trackingoperation. The Vth tracking operation is an operation to search for avoltage corresponding to the intersection of overlapping thresholddistributions by measuring the number of on-cells (or off-cells) whilefinely changing the voltage to applied to the selected word line WL. Theoptimal read voltage is determined based on the intersection, and a readoperation is performed again.

When the history table does not designate the last index in step S13 (NOin step S13), the controller 200 selects the next index in the shifttable (step S16). Then, the controller 200 issues a shift read commandbased on the selected index (step S17). At such time, for example, thecommand sequence issued by the controller 200 includes informationindicating shift amounts ΔV, and the NAND flash memory 100 determinesread voltages VCGRV based on this information. In the above-describedexample, when word line WL2 of block BLK0 is selected, voltages(VA+ΔVA8), (VB+ΔVB8), (VC+ΔVC8), . . . , and (VG+ΔVG8) are used as readvoltages VCGRV.

Then, the controller 200 repeats the shift read (steps S16 and S17)until the selected index reaches the last value (YES in step S13) orerror correction is successful (YES in step S18).

When the ECC circuit 260 succeeds in error correction (YES in step S18),the controller 200 updates the history table in the memory 220 (stepS19). Namely, in the history table, the index value concerning the wordline WL of the logical block LB to be read is changed to the valueselected in the latest step S16. The controller 200 also updatesinformation in the register as shown in FIG. 11 by using, for example,the set feature command. FIG. 11 shows an example in which a shift readon word line WL2 is successful when the index is “8.” As shown in FIG.11 , the shifts amount ΔVA7, ΔVB7, . . . , and ΔVG7 for word line WL2are updated to shifts amounts ΔVA8, ΔVB8, . . . , and ΔVG8,respectively.

Subsequently, the controller 200 compares the index selected in step S16with the second reference value described in FIG. 5 (step S20). When theindex is larger than the second reference value (YES in step S20) , thecontroller 200 reserves a copy of the logical block as in step S14 (stepS21), and ends the read operation.

On the other hand, when the index is equal to or smaller than the secondreference value (No in step S20), the controller 200 compares the indexwith the first reference value described in relation to FIG. 5 (stepS22). When the index is larger than the first reference value (YES instep S22), the controller 200 reserves a refresh read operation (stepS23). Namely, in the flag table in the memory 220, a refresh read flagis set for the logical block LB corresponding to the physical block BLKto be read. Then, the read operation ends. When the index is equal to orsmaller than the first reference value (NO in step S22), the (controller200 ends the read operation without setting a refresh read flag.

1.2.2 Refresh Read Operation

A flow of the refresh operation by the controller 200 will be describedwith reference to FIG. 12 . FIG. 12 is a flowchart of the refresh readoperation. The operation of the controller 200 in FIG. 12 is executedmainly by control of the processor 230, for example.

The controller 200 confirms the flag table in the memory 220 immediatelyafter data reading described in relation to FIG. 8 , or an unoccupiedtime during processing of the controller 200 after data reading. Then,the controller 200 executes a refresh read operation on a logical blockLB for which “1” is set as a refresh read flag.

Namely, as shown in FIG. 12 , the controller 200 initializes the numberof refresh reads (RD_count=0) (step S30). Subsequently, the controller200 compares the number of refresh reads with the maximum number ofrefresh reads (step S31). The maximum number of refresh reads is one ora larger value. Like the shift table and the history table, the maximumnumber of refresh reads is, for example, read from the NAND flash memory100 and held in the memory 220.

When the number of refresh reads is smaller than the maximum number ofrefresh reads (YES in step S31), the controller 200 issues a refreshread command, and transmits it to the NAND flash memory 100 (step S32).The controller 200 also increments the number of refresh reads(RD_count=RD_count+1) (step S33). Then, the processing returns to stepS31.

When the number of refresh reads is equal to or larger than the maximumnumber of refresh reads in step S31 (NO in step S31), the controller 200updates the history table (step S34), and the refresh read operationends. Unlike in step S19, the processor 230 updates an index in thehistory table to a smaller value in step S34. In other words, theprocessor 230 updates an index to an index corresponding to smallershift amounts, i.e., an index for dealing with a smaller influence ofthe inter-cell interference effect. For example, the processor 230updates an index to the one before the first reference value.

FIG. 13 is a timing chart of a data signal DQ and a ready/busy signal RBin the normal read operation and the refresh read operation. Signal DQis a signal transmitted and received between the controller 200 and theNAND flash memory 100, and a command, an address, data, and the like aretransmitted and received as signal DQ. Signal RB is a signal transmittedfrom the NAND flash memory 100 to the controller 200, and is brought tothe “H” level (ready state) when the NAND flash memory 100 can receive acommand from the controller 200, and to the “L” level (busy state) whenthe NAND flash memory 100 cannot receive a command.

As shown in FIG. 13 , the following command sequence that provides aninstruction to perform a normal read is transmitted from the controller200 to the NAND flash memory 100 at time t0:

<00h><ADD><ADD><ADD><ADD><ADD><30h>

Namely, address input is announced by command “00h,” and address ADD isinput over five cycles. This address ADD designates a physical block BLKand page to be read. Upon receipt of command “30h,” the sequencer 170starts data reading from a memory cell, and the NAND flash memory 100changes to the busy state.

Upon completion of data reading from the memory cell, the NAND flashmemory 100 changes to the ready state. Then, read data is transmitted tothe controller 200 in synchronization with a clock (not shown)transmitted from the controller 200.

When providing an instruction to perform a shift read instead of thenormal read the following command sequence, for example, is transmittedfrom the controller 200 to the NAND flash memory 100:

<X0h><DAC><00h><ADD><ADD><ADD><ADD><ADD><30h>

Namely, designation of read voltages is announced by command “X0h,” andread voltages VCGRV are designated by data “DAC.” Data “DAC” mayindicate the read voltages VCGRV themselves, or may indicate shiftamounts from the default values. After that, the same command sequenceas that of the normal read is transmitted.

Here, let us assume that a refresh read is reserved for the logicalblock LB from which data was read during time t0 to t1. In this case,after read data is transmitted from the NAND flash memory 100 to thecontroller 200, the following command sequence that provides aninstruction to perform a refresh read is transmitted from the controller200 to the NAND flash memory 200 at time t2.

<XXh><00h><ADD><ADD><ADD><ADD><ADD><30h>

This command sequence is a command obtained by adding a prefix command“XXh” before the command sequence that provides an instruction toperform a normal read transmitted at time t0. Command “XXh” provides aninstruction to execute a refresh read on a memory cell (block BLK)designated by the subsequent address. Therefore, address “ADD” in fivecycles designates the same address as the address “ADD” in the commandsequence of the latest normal read or shift read.

In response to this command sequence, the sequencer 170 starts a refreshread, and the NAND flash memory 100 changes to the busy state. Afterthat, every time the NAND flash memory 100 returns to the ready state,the controller 200 instructs the NAND flash memory 100 to repeat arefresh read until the number of refresh reads reaches the maximumnumber of refresh reads. Note that, unlike in the normal read, read datais not transmitted to the controller 200 in the refresh read as shown inFIG. 13 .

1.2.3 Operation of NAND Flash Memory

Next, an operation of the NAND flash memory 100 performed during each ofthe above-described normal read, shift read, and refresh read will bedescribed while focusing particular attention on the memory cell array110.

First, the operation during the normal read and the shift read will bedescribed with reference to FIG. 14 . FIG. 14 shows, as an example, acase where data is read from memory cell transistor MT2 in string unitSU0 of physical block BLK0.

As shown in FIG. 14 , voltage VSG (for example, 4.3V) is applied toselection gate lines SGD0 and SGS0. Voltage VSG is a voltage that turnson selection transistors ST1 and ST2.

Voltages VREAD_DD and VREAD_DS (for example, 7.8V) are applied to dummyword lines WLDD and WLDS, respectively. Voltages VREAD_DD and VREAD_DSare voltages that turn on dummy transistors DTD and DTS, respectively.

Voltage VREAD (for example, 8.0V) is applied to the non-selected wordlines WL0, WL4, W15, . . . , and W17 not adjacent to the selected wordline WL2. Voltage VREADK (for example, 8.2V) is applied to thenon-selected word lines WL1 and WL3 adjacent to the selected word lineWL2. Voltages VREAD and VREADK are voltages that turn on memory celltransistors MT regardless of held data.

Read voltages VCGRV are applied to the selected word line WL2. Forexample, when the lower page is read, voltages (VA+ΔVA) and (VE+ΔVE) areused as voltages VCGRV. ΔVA and ΔVE are shift amounts from the defaultvalues.

A pre-charge voltage Vpre (for example, 0.7V) is applied to the bitlines BL, and the source line SL is brought to 0V. Accordingly, readoperation AR is performed during a period in which voltage (VA+ΔVA) isapplied as voltage VCGRV, and read operation ER is performed during aperiod in which voltage (VE+ΔVE) is applied as voltage VCGRV.

Specifically, when memory cell transistor MT2 is turned on, a cellcurrent flows from the bit line BL to the source line SL, and when it isoff, a cell current does not flow (Only a slight leak current flows).The sense amplifier 140 senses the current that flows in the bit lineBL, or the potential of the bit line BL, determines whether the dataheld in memory cell transistor MT2 is “0” or “1,” and holds the data inan internal latch circuit. After the sense amplifier 140 performs anecessary arithmetic operation on the read data, the data is transmittedto the controller 200.

In the non-selected string units SU1 to SU3, 0V is applied to theselection gate lines SGD1 to SGD3 and SGS1 to SGS3, and selectiontransistors ST1 and ST2 are turned off.

Next, the operation during the refresh read will be described withreference to FIG. 15 . As shown in FIG. 15 , voltage VREAD is applied toall the word lines WL0 to WL7. All the memory cell transistors MT0 toMT7 in block BLK0 are thereby turned on regardless of held data. Theother part of the operation is the same as the operation performedduring the normal read and the shift read. Note, however, that therefresh read is not an operation for reading data from a memory celltransistor MT, and the sense amplifier 140 need not sense the potentialchange of the bit line BL as read data. In the refresh read, the bitline BL need not necessarily have the pre-charge potential as long asthe memory cell transistor MT is provided with a voltage stress similarto a read disturb and, for example, 0V or any other fixed voltage may beapplied to the bit lines BL.

1.2.4 Specific Examples

Next, specific examples of the case where a refresh read is executed andthe case where a block copy is executed in FIG. 8 will be brieflydescribed.

<Refresh Read>

First, the case where a refresh read is performed will be described withreference to FIG. 16 . Described below as an example is the case wherelower page data is read from memory cell transistor MT2 in physicalblock BLK0 corresponding to logical block LB0.

First, the NAND flash memory 100 performs a normal read in response to anormal read command from the controller 200 (step S11). At this time, asdescribed in relation to FIGS. 6 and 7 , the index corresponding to wordline WL2 of logical block LB0 is “7”; therefore, shift amounts ΔVA7 andΔVE7 held in the shift table are selected. Namely, in the NAND flashmemory 100, the driver circuit 130 applies voltages (VA+ΔVA7) and(VE+ΔVE7) to selected word line WL2 in respective read operations AR andER.

When data read in step S11 is transmitted to the controller 200, the ECCcircuit 260 attempts error correction. When the ECC circuit 260 fails inthe error correction (step S12), the controller 200 selects the nextindex, i.e., (7+1)=8 (step S16). Then, the controller 200 issues a shiftread command, and the NAND flash memory 100 executes a shift read (stepS17). At this time, the controller 200 instructs the NAND flash memory100 to use shift amounts ΔVA8 and ΔVE8 corresponding to index value=8 inthe shift table. In accordance with the instruction, in the NAND flashmemory 100, the driver circuit 130 applies voltages (VA+ΔVA8) and(VE+ΔVE8) to selected word line WL2 in respective read operations AR andER.

When read data read in step S17 is transmitted to the controller 200,the ECC circuit 260 attempts error correction again. When the ECCcircuit 260 succeeds in error correction (step S18), the controller 200updates the history table in the memory 220 (step S19). Namely, thecontroller 200 updates the index corresponding to word line WL2 from “7”to “8.” That is, the values shifted by the shift amounts correspondingindex=8 will be hereafter used as default values of read voltages VCGRV.

Subsequently, the controller 200 compares the index with the secondreference value (step S20). The index was updated from “7” to “8” instep S19. The second reference value is “10.” Therefore, the index issmaller than the second reference value. Consequently, a block copy isnot performed.

Subsequently, the controller 200 compares the index with the firstreference value (step S22). The index is “8” as mentioned above, and thefirst reference value is “7.” Therefore, the index is larger than thefirst reference value. Accordingly, the controller 200 sets a refreshflag for logical block LB0 in the flag table in the memory 220 (stepS23).

After that, the controller 200 issues a refresh read command in anunoccupied time during processing and, in response thereto, the NANDflash memory 100 executes a refresh read until the number of refreshreads reaches the maximum number of refresh reads (step S32). Namely,voltage VREAD is applied to all the word lines WL in the selected blockBLK. The controller 200 also updates the history table in the memory 220(step S34). Namely, the index is updated from “8” to, for example, theindex “6” which is the one before the first reference value.

<Block Copy>

Next, the case where a block copy is executed will be described withreference to FIG. 17 . Described below as an example is the case wherelower page data is read from memory cell transistor MT7 in any one ofphysical blocks BLK4 to BLK7 corresponding to logical block LB1.

As in the case of FIG. 16 , the NAND flash memory 100 first performs anormal read (step S11). At this time, as described in relation to FIGS.6 and 7 , the index corresponding to word line WL7 of logical block LB1is “10”; therefore, shift amounts ΔVA10 and ΔVE10 held in the shifttable are selected.

When error correction on read data read in step S11 ends in failure(step S12), the controller 200 selects the next index, i.e., (10+1)=11(step S16). Then, the controller 200 issues a shift read command, andthe NAND flash memory 100 executes a shift read (step S17). The shiftamounts used at this time are shift amounts ΔVA11 and ΔVE11corresponding to index=11.

When error correction of read data read in step S17 is successful (stepS18), the controller 200 updates the history table in the memory 220(step S19). Namely, the controller 200 updates the index correspondingto word line WL7 from “10” to “11.”

Subsequently, the controller 200 compares the index with the secondreference value (step S20). The index was updated from “10” to “11” instep S19. The second reference value is “10.” Therefore, the index islarger than the second reference value. Accordingly, the controller 200sets a block copy flag for logical block LB1 in the flag table in thememory 220 (step S21).

After that, the controller 200 issues a block copy command in anunoccupied time during processing and, in response thereto, the NANDflash memory 100 executes a block copy (step S42). Namely, the NANDflash memory 100 copies the effective data held in the physical blocksBLK4 to BLK7 corresponding to logical block LB1 to physical blocks BLK8to BLK11 from which data has been erased. Then, the NAND flash memory100 erases all data in physical blocks BLK4 to BLK7, and the controller200 thereafter assigns physical blocks BLK8 to BLK11 to logical blockLB1.

The controller 200 also updates the history table (step S43). Namely,the controller 200 initializes the indexes corresponding to logicalblock LB1. In the case shown in FIG. 17 , the index values of all theword lines WL0 to WL7 are changed to “0.” “0” means, for example, shiftvalue=“0.” Alternatively, the indexes of all the word lines WL0 to WL7may be changed to the minimum value “1.”

1.3 Advantage According to Present Embodiment

According to the present embodiment, operation reliability of the memorysystem can be improved. This advantage will be described with referenceto FIG. 18 . FIG. 18 shows changes in the threshold distribution ofmemory cells.

The first state in FIG. 18 is threshold distribution immediately afterdata writing. The first state is an ideal state in which the thresholddistributions are apart from one another. The second state is thresholddistribution after a certain time has elapsed from the first state. Asshown in FIG. 18 , threshold distributions are shifted to the lowvoltage side due to the above-described inter-cell interference effect.This is because, for example, electrons injected in the chargeaccumulation layer move to the region between memory cell transistors,and the number of electrons in the charge accumulation layer of thememory cell transistor MT substantially decreases. Therefore, thecontroller 200 deals with this inter-cell interference effect by a shiftread.

In the present embodiment, to deal with the inter-cell interferenceeffect, a refresh read is performed when shift amounts of read voltagesVCGRV (i.e., index) exceed the first reference value. The refresh readis an operation to apply voltage VREAD to all the word lines WL asdescribed in FIG. 15 . A voltage stress is thereby applied to the memorycell transistors MT, and threshold distributions are shifted to the highvoltage side as in the third state shown in FIG. 18 . Namely, thechanges of threshold distributions from the first state to the secondstate can be at least partly canceled by the refresh read.

According to the present embodiment, threshold distribution of memorycell transistors MT (i.e., reliability of held data) is determined basedon the index, as described above. When the threshold distribution(index) falls lower than a fixed reference value, a refresh read isperformed. Reliability of data held in memory cell transistors MT isthereby improved, and the number of error bits at the time of readingcan be reduced. As mentioned above, reading of data from the memory cellarray 110 to the sense amplifier 140 is not necessary in the refreshread operation and, even if data is read, the data is not needed to betransmitted to the controller 200. This is because the refresh readoperation is an operation to shift threshold distributions by applying avoltage stress to the memory cell transistors MT, rather than anoperation to read data. Therefore, the refresh read operation can becalled, for example, a threshold distribution shift operation, athreshold correction operation, a stress application operation, or aread disturb application operation.

Furthermore, when the number of error bits becomes very large, andcorrection by the FCC circuit 260 becomes almost impossible, thecontroller 200 performs a copy operation to copy the logical block LB.However, in general, this copy operation is performed on a very largenumber of physical blocks (for example, 128 blocks BLK), which places aheavy load on the memory system 1.

According to the present embodiment, however, threshold distributionsare corrected by the above-described refresh read. Therefore, thefrequency of the copy operation can be reduced, and operationreliability of the memory system can be improved.

In the present embodiment, the case where threshold distributions areshifted to the low voltage side due to the inter-cell interferenceeffect has been described as an example. However, the inter-cellinterference effect may have various mechanisms. For example, theinter-cell interference effect may be caused by threshold changeamount(s) of memory cell transistor(s) MT(i−1) and/or MT (i+1) adjacentto memory cell transistor MTi (i is an integer equal to or larger than0) which are produced after memory cell transistor MTi passes a programverification. In this case, the threshold voltage of memory celltransistor MTi is shifted to the high voltage side. The lower (“Er”state) the threshold voltage of memory cell transistor MTi to be readis, and the higher the threshold voltage(s) of the adjacent memory celltransistor(s) MT(i−1) and/or MT (i+1) is (are), the larger the changeamount of the threshold voltage of memory cell transistor MTi tends tobe.

In the above-described embodiment, from when a refresh read is reservedin step S23 of FIG. 8 to when the refresh read is actually performed, adata write operation and erase operation are not executed on the pageread in steps S11 and S17. This is because data writing is performed onmemory cell transistors MT from which data has been erased, and data isnot written in the page in which data has been written. In addition, ifan erase operation is executed, a refresh read no longer needs to beperformed on the page (and the block BLK including the page). Namely,after a refresh read is reserved, the refresh read is performed unlessan erase operation or a write operation (which includes a data eraseoperation) on the page is executed. However, a read operation may beperformed on the page and the block BLK. In addition, a write operationand an erase operation on the other blocks BLK may be performed, and awrite operation may be performed on the other string units SU.

Therefore, the shift table may take into account such an influence ofthe inter-cell interference effect. For example, the shift table mayhave some indexes associated with shift amounts set by particularlyfocusing attention to the shift to the high voltage side as described inthe above embodiments, some other indexes associated with shift amountsset by particularly focusing attention to the shift to the low voltageside, and some other indexes associated with shift amounts set byfocusing attention to both of the shifts.

2. Second Embodiment

The memory system according to the second embodiment will be described.In the present embodiment, a desired index is set after performing arefresh read and then updating the history table in the firstembodiment, and whether data can be read with the index is confirmed. Inthe following description, only the matters different from the firstembodiment will be described.

2.1 Refresh Read Operation

The refresh read operation according to the present embodiment will bedescribed with reference to FIG. 19 . FIG. 19 is a flowchart of therefresh read operation, and corresponds to FIG. 12 described in thefirst embodiment. As in FIG. 12 , the operation of the controller 200 inFIG. 19 is executed mainly by control of the processor 230, for example.

As shown in FIG. 19 , the controller 200 initializes the number ofrefresh reads in step S30 (RD_count=0), and initializes the number ofrepeats of the shift read (Repeat_count=0) (step S35). Subsequently, thecontroller 200 compares the number of repeats of the shift read with themaximum number of shift reads (step S36). The maximum number of shiftreads is one or a larger value, and is, for example, read from the NANDflash memory 100 and held in the memory 220 like the shift table and thehistory table.

When the number of repeats of the shift read is equal to or larger thanthe maximum number of shift reads in step S36 (NO in step S36), thecontroller 200 ends the refresh read operation.

On the other hand, when the number of repeats is smaller than themaximum number of shift reads in step S36 (YES in step S36), thecontroller 200 compares the number of refresh reads with the maximumnumber of refresh reads (step S31).

When the number of refresh reads is smaller than the maximum number ofrefresh reads (YES in step S31), a refresh read is repeated until thenumber of refresh reads reaches the maximum number as in the firstembodiment (steps S32 and S33).

When the number of refresh reads is equal to or larger than the maximumnumber of refresh reads in step S31 (NO in step S31), the controller 200instructs NAND flash memory 100 to perform a shift read with a targetindex (step S37). The target index is a value equal to or smaller thanthe first reference value, and a shift read is performed by using shiftamounts corresponding to the target index in step S37.

When the ECC circuit 260 succeeds in error correction on data read instep S37 (YES in step S38), the processor 230 updates the index value inthe history table to the target index used in S37 (step S34), and endsthe refresh read operation.

On the other hand, when error correction ends in failure (NO in stepS38), the controller 200 increments the number of repeats of the shiftread (Repeat_count=Repeat_count+1) (step S39), and the processingreturns to step S36.

2.2 Specific Examples

A specific example of the refresh read operation according to thepresent embodiment will be described with reference to FIG. 20 . FIG. 20corresponds to FIG. 16 described in the first embodiment, and shows thecase where lower page data is read from memory cell transistor MT2 in aphysical block BLK corresponding to logical block LB0.

As shown in FIG. 20 , the processing until step S23 is the same as thatin FIG. 16 described in the first embodiment. Namely, the index for wordline WL0 of logical block LB0 is changed to “8” to exceed the firstreference value, and a refresh read is reserved.

In step S32, a refresh read is repeated until the number of refreshreads reaches the maximum number of refresh reads (step S32) and then,in step S37, a shift read is performed. Here, let us assume that theprocessor 230 selects “3” as the target index, for example. In the NANDflash memory 100, the driver circuit 130 applies voltages (VA+ΔVA3) and(VE+ΔVE3) to selected word line WL2 in respective read operations AR andER. Namely, at this point in time, information in the history table(index=“8”) does not match the index used in the NAND flash memory 100(target index=“3”).

As a result of performance of the combination of the refresh read instep S32 and the shift read in step S37 once or multiple times, the ECCcircuit 260 succeeds in error correction (step S38). The processor 230then updates the index value in the history table from “8” to the targetindex “3” (step S34), and the refresh read operation is completed.

2.3 Advantage According to Present Embodiment

According to the present embodiment, operation reliability of the memorysystem can be further improved. Namely, according to the presentembodiment, a shift read is performed after refresh read(s) and, afterconfirming that data can be correctly read, the index in the historytable is updated. Therefore, reliability of the history table updated instep S34 can be improved.

In addition, when data cannot be correctly read by the target index, thehistory table is not updated. Therefore, application of the presentembodiment is preferable when the controller 200 is expected to set avalue significantly smaller than the first reference value as the targetindex, i.e., when the shift amounts for compensating for an influence ofthe inter-cell interference effect are desired to be updated torelatively small values.

Even when error correction ends in failure by setting a valuesignificantly smaller than the first reference value as the targetindex, the threshold distribution is expected to be shifted to the highvoltage side to a certain extent by refresh read(s). Therefore, theindex may be updated to a value (“7”) equal to the first reference valueas in the first embodiment, or a close value (such as “5” or “6”) in thehistory table.

3. Third Embodiment

Next, the memory system according to toe third embodiment will bedescribed. In the present embodiment, the refresh read operationdescribed in the above second embodiment is executed immediately afterthe data read operation. In the following description, only the mattersdifferent from the first and second embodiments will be described.

3.1 Read Operation and Refresh Read Operation

FIG. 21 is a flowchart of the read operation and refresh read operationaccording to the present embodiment. As shown in FIG. 21 , the refreshread operation according to the present embodiment is executed as a partof the read operation. In other words, a refresh read is performed everytime a shift read ends in failure.

Namely, as shown in FIG. 21 , when the data is failed to be corrected inthe shift read in step S17 (NO in step S18), the controller 200 comparesthe index with the first reference value (step S22). When the index isequal to or smaller than the first reference value (NO in step S22), theprocessing returns to the judgment in step S13. If the index is largerthan the first reference value (YES in step S22), the controller 200executes a refresh read operation (step S50).

The refresh read operation S50 is almost the same as that in FIG. 19described in the second embodiment, but is different from that in thesecond embodiment in the following respects:

-   -   When the number of repeats of the shift read with the target        index reaches the maximum value in step S36 (NO in step S36),        the processing returns to step S13.    -   When error correction succeeds in the shift read with the target        index in step S37 (YES in step S38), the process proceeds to        step S19, and the index in the history table is updated to the        target index.

After step S19, the index value is compared with the second referencevalue in step S20, and a copy of the logical block is reserved inaccordance with the comparison result (step S21), whereby the readoperation is completed.

3.2 Advantage According to Present Embodiment

According to the present embodiment, when a shift read results infailure (NO in steps S17 and S18), and the index is equal to or largerthan the first reference value, a refresh read is executed immediatelyafter the shift read. Accordingly, a refresh read operation may beperformed as a part of a data read operation.

The shift read in step S37 of the second embodiment is merely anoperation for confirming whether the target index is appropriate or not.Data read and error-corrected in step S17 is transmitted to the hostapparatus 300.

In contrast, according to the present embodiment, when the shift read instep S17 ends in failure, and the shift read in step S37 is successful,data error-corrected in steps S37 and S38 is transmitted to the hostapparatus 300. The number of repeats compared in step S36 is not thenumber of repeats of the shift read executed in step S17, but the numberof repeats of the shift read executed in step S37.

4. Modifications, etc.

As described above, the memory system 1 according to the above-describedembodiments includes: a semiconductor memory including a memory cellarray including a first memory cell and a second memory cell configuredto hold data and are coupled to a first word line and a second wordline, respectively; and a controller configured to control thesemiconductor memory. The memory system is capable of executing a firstoperation (read operation) and a second operation (refresh readoperation) after the first operation. In the first operation (readoperation), the controller issues a first command sequence (00h-ADD-30h)and, in response to the first command sequence, the semiconductor memoryapplies a first voltage (VCGRV) to the first word line (WL2 in FIG. 14 )and applies a second voltage (VREAD) to the second word line (WL4-7 inFIG. 14 ) to read data from the first memory cell (MT1 in FIG. 14 ). Theread data is transmitted from the semiconductor memory to the controller(normal read or shift read). In the second operation (refresh readoperation), the controller issues a second command sequence (XXh+1stsequence) and, in response to the second command sequence, thesemiconductor memory applies a third voltage (VREAD or VREADK or plus α)to the first word line (WL2 in FIG. 15 ) and applies a fourth voltage(VREAD or VREAD plus α) to the second word line (WL4-7 in FIG. 15 ). Thedata held in the memory cell array is left untransmitted to thecontroller (refresh read). The second operation (refresh read operation)is executed after the first operation (read operation) withoutintervention of a write operation and erase operation on the firstmemory cell. The third voltage (VREAD) turns on the first memory cellregardless of data held in the first memory cell, and the second voltage(VREAD or VREADK) and the fourth voltage (VREAD) turn on the secondmemory cell regardless of data held in the second memory cell.

This configuration enables correction of threshold changes of memorycells involved with an elapse of time, thereby improving reliability ofthe operations of the memory system 1. The above-described embodimentsare mere examples, and various modifications are possible. For example,described in the above-described embodiments as an example is the casewhere voltage VREAD is applied with no word line WL selected in arefresh read. However, for example, a dummy word line may be selected.Such an example is shown in FIGS. 22 and 23 . FIGS. 22 and 23 arecircuit diagrams of a block BLK in a refresh read.

In the example of FIG. 22 , the dummy word line WLDD on the drain sideis selected, and voltage VCGRV is applied thereto. In the example ofFIG. 23 , the dummy word line WLDS on the source side is selected, andvoltage VCGRV is applied thereto. The potentials of the other rises areas described in relation to FIG. 15 . The value of voltage VCGRV is notparticularly limited. The value of voltage VCGRV may be any value thatcan provide memory cell transistors MT with a read disturb, and may be,for example, one of voltages VA, VB, . . . , and VG, or may be multiplevalues (such as VA and VE for lower voltage page reading, VB, VD, and VFfor middle page reading, and VC and VG for upper page reading) as innormal page data reading. In this case, the controller 200 issues, forexample, the following command sequence:

<YYh><00h><ADD><ADD><ADD><ADD><ADD><30h>

Namely, prefix command “YYh” is added at the top of the normal readsequence. This “YYh” provides an instruction to select a dummy wordline. At this time, “ADD” indicates the block BLK designated in thelatest normal read or shift read as in the above-described embodiments.Alternatively, “ADD” may include information indicating which of thedummy word line WL_DD on the drain side and the dummy word line WL_DS onthe source side should be selected as well as information indicating theblock BLK.

The refresh read may be the same operation as the normal read operationas shown in FIG. 14 . Namely, the refresh read may be an operation inwhich one word line WL is selected and voltage VCGRV is applied thereto,and voltage VREAD is applied to the other non-selected word lines WL.The value of VCGRV in this case may also be one of voltages VA, VB, . .. , and VG (or values obtained by adding shift amounts designated in theshift table to those voltages), or may be multiple values as in normalpage data reading. In this case, the controller 200 issues, for example,the following command sequence:

<XYh><00h><ADD><ADD><ADD><ADD><ADD><30h>

Namely, prefix command “XYh” is added at the top of the normal readsequence. This “XYh” announces a refresh read. At this time, “ADD”indicates a block BLK and page designated in the latest normal read orshift read as in the above-described embodiments. Therefore, the NANDflash memory 100 that has received this command sequence applies avoltage to the word lines WL like in the normal read or shift read, butdoes not perform, by the sense amplifier 140, a data sense operation ordata transfer to the controller 200.

Furthermore, the refresh read may be executed with a plurality of stringunits SU selected. FIG. 24 shows, as an example, the case where allstring units SU0 to SU3 in a selected block BLK are selected in therefresh read of FIG. 14 described in the first embodiment. As shown inFIG. 24 , voltage VSG is applied not only to selection gate line SGD0,but also to selection gate lines SGD1 to SGD3, and selection transistorsST1 in string units SU1 to SU3 are turned on. In this case, for example,the following command sequence is transmitted from the controller 200 tothe NAND flash memory 100:

<XXh><YZh><00h><ADD><ADD><ADD><ADD><ADD><30h>

Namely, in the second cycle, for example, of the command sequence of arefresh read described in the first embodiment, a command “YZh” forselecting a plurality of string units SU is transmitted. Needless tosay, the command sequence is not limited to this sequence, and a similarinstruction may be transmitted by, for example, replacing “XXh” withanother command.

This is applicable to the case where dummy word line WLDD or WLDS isselected which is described in relation to FIGS. 22 and 23 , and thecase where one word line WL is selected. In those cases, command “YZh”may be issued in, for example, the second cycle. FIG. 24 shows the casewhere all string units SU0 to SU3 in a block BLK are selected; however,all the string units need not be necessarily selected, and some stringunits SU may he selected. In this case, for example, another command maybe prepared instead of command “YZh,” or a command designating whichstring units SU should be selected may be issued after command “YZh.”

Moreover, a refresh read may be performed on another physical block BLKbelonging to the same logical block LB. This is because indexes areassigned to each logical block LB in the example (FIG. 6 ) of theabove-described embodiments. However, indexes need not necessarily beassigned to each logical block LB, and may be assigned to each physicalblock BLK, or each set of physical blocks.

In addition, in the example of FIG. 6 , an index is assigned to eachword line WL. However, one index may be assigned to each physical blockBLK, or may be assigned to each group of word lines WL by grouping aplurality of word lines WL. Such an example is shown in FIGS. 25 and 26. FIGS. 25 and 26 are conceptual diagrams of the history table. In theexample of FIG. 25 , two adjacent word lines WL are grouped. Indexes arerespectively assigned to those four groups. Furthermore, in the exampleof FIG. 26 , the word lines WL are divided into three groups: two (WL0and WL1) in the lower layer of the NAND string 10, two (WL6 and WL7) inthe upper layer thereof, and four (WL2 to WL5) provided therebetween.Then, indexes are assigned respectively to those three groups.

Described in the above embodiments as an example is the case where thesame voltage (for example, VREAD) as that applied to the non-selectedword lines WL in the normal read and shift read, is applied to the wordlines WL in the refresh read. However, the voltage applied to the wordlines WL in the refresh read may be larger than that applied in thenormal read and shift read. Such a case will be described with referenceto FIGS. 27 and 28 . FIGS. 27 and 28 are timing charts of selected wordline WLi, non-selected word line WL(i±1), the other non-selected wordlines WL, dummy word lines WLDD and WLDS, and selection gate lines SGDand SGS in the normal read or shift read and in the refresh read; FIG.27 shows the case where the refresh read is not performed on thenon-selected string units, and FIG. 28 shows the case where the refreshread is also performed on the non-selected string units.

As shown in FIGS. 27 and 28 , in the normal read or shift read, voltageVREADK is applied to non-selected word line WL(i±1), voltage VREAD isapplied to one other non-selected word lines WL, and voltages VREAD_DDand VREAD_DS are applied to dummy word lines WLDD and WLDS. In contrast,in the refresh read, voltage (VREAD+ΔVREAD) is applied to all the wordlines WL. ΔVREAD is a positive value, and is, for example, 1.0V. Thethreshold values of memory cell transistors can be thereby moreeffectively shifted to the high voltage side.

Note that the voltage applied to word lines WLi and WL(i±1) may be(VREADK+ΔVREADK) instead of (VREAD+ΔVREAD). ΔVREAD is a positive value,and (VREADK+ΔVREADK) may be the same value as or a different value from(VREAD+ΔVREAD). Similarly, the voltages applied to selection gate linesSGD and SGS may be (VREAD_DD+ΔVREAD_DD) and (VREAD_DS+ΔVREAD_DS).ΔVREAD_DD and ΔVREAD_DS are positive values, and (VREAD_DD+ΔVREAD_DD)and (VREAD_DS+ΔVREAD_DS) may be the same value as, or a different valuefrom (VREAD+ΔVREAD).

In addition, the voltage application period may be lengthened instead ofincreasing applies voltages as in the cases of FIGS. 27 and 28 . LikeFIGS. 27 and 28 , FIGS. 29 and 30 are timing charts of voltages appliedto each line; FIG. 29 shows the case where the refresh read is notperformed on the non-selected string unit(s), and FIG. 30 shows the casewhere the refresh read is also performed on the non-selected stringunit(s).

As shown in FIGS. 29 and 30 , in the refresh read of the presentmodification, voltage VREADK is applied to word line WL(i±1), voltageVREAD is applied to the other word lines WL, and voltages VREAD_DD andVREAD_DS are applied to dummy word lines WLDD and WLDS, as in the normalread or shift read. However, the period during which those voltagesVREAD, VREADK, VREAD_DD, and VREAD_DS are applied is longer in therefresh read (Δt2) than in the normal read or shift read (Δt1). Forexample, Δt2 is the double of Δt1 or longer. The present method alsoperforms the same advantage as in the cases of FIGS. 27 and 28 .

Needless to say, the method of FIGS. 27 and 28 may be combined with themethod of FIGS. 29 and 30 . Namely, the voltage applied to the wordlines in the refresh read may be made higher than in the normal read andshift read while making Δt2 larger-than Δt1. That is, thresholddistributions can be more efficiently corrected by increasing theapplied voltage as well as lengthening the application period. Insteadof making the voltage applied to the word lines in the refresh readlarger than in the normal read and shift read, Δt2 may be made smallerthan Δt1. Namely, the time required for the refresh read may beshortened by increasing the applied voltage.

The magnitude of the voltage applied to the word lines in the refreshread and the length of the application period may be designated in thecommand sequence of the refresh read, or may be set by the set featurecommand. The following is an example of the command sequence:

<XXh><ZXh><VALUE><00h><ADD><ADD><ADD><ADD><ADD><30h>

After the prefix command “XXh” described above, command “ZXh” and data“VALUE” are transmitted from the controller 200 to the NAND flash memory100. Command “ZXh” designates an increase of the voltage or lengtheningof the voltage application period in the refresh read, and “VALUE”indicates a voltage value or an application period.

Described as an example, in the above embodiments is the case where thevoltage shift amounts in the shift read are designated in the commandsequence; however, the voltage shift amounts may be preset in the NANDflash memory 100 by the set feature command. Described as arc example,in the above embodiments is the case where each memory cell transistorMT holds 3-bit data. However, the embodiments may be applied to the casewhere each memory cell transistor MT holds 2-bit data or 4 or larger bitdata.

Note that in each embodiment concerning the present embodiments,

(1) When the memory cell holds 2-bit data (“Er”, “A”, “B”, and “C”), thevoltage applied to the selected word line in the reading operation of Alevel may range from, for example, 0 V to 0.55 V. However, the presentembodiments are not limited to this, and the voltage may be set withinany one of the ranges of 0.1 V to 0.24 V, 0.21 V to 0.31 V, 0.31 V to0.4 V, 0.4 V to 0.5 V, and 0.5 V to 0.55 V.

The voltage applied to the selected word line in the reading operationof B level may range from, for example, 1.5 V to 2.3 V. However, thevoltage is not limited to this and may be set within any one of theranges of 1.65 V to 1.8 V, 1.8 V to 1.95 V, 1.95 V to 2.1 V, and 2.1 Vto 2.3 V.

The voltage applied to the selected word line in the reading operationof C level may range from, for example, 3.0 V to 4.0 V. However, thevoltage is not limited to this and may be set within any one of theranges of 3.0 V to 3.2 V, 3.2 V to 3.4 V, 3.4 V to 3.5 V, 3.5 V to 3.6V, and 3.6 V to 4.0 V.

A time (tR) of the reading operation may be set within the range of, forexample, 25 μs to 38 μs, 38 μs to 70 μs, or 70 μs to 80 μs.

(2) A writing operation may include a program operation and a verifyoperation. In the writing operation, the voltage first applied to theselected word line in the program operation may range from, for example,13.7 V to 14.3 V. The voltage is not limited to this and may be setwithin any one of the ranges of, for example, 13.7 V to 14.0 V and 14.0V to 14.6 V.

The voltage first applied to the selected word line when write-accessingan odd-numbered word line and the voltage first applied to the selectedword line when write-accessing an even-numbered word line may bedifferent.

If the program operation may be ISPP (Incremental Step Pulse Program),the voltage of step-up may be, for example, 0.5 V.

The voltage applied to an unselected word line may be set within therange of, for example, 6.0 V to 7.3 V. However, the voltage is notlimited to this and may be set within the range of, for example, 7.3 Vto 8.4 or set to 6.0 V or less.

The pass voltage to be applied may be changed depending on whether theunselected word line is an odd-numbered word line or an even-numberedword line.

A time (tProg) of the writing operation may be set within the range of,for example, 1,700 μs to 1,800 μs, 1,800 μs to 1,900 μs, or 1,900 μs to2000 μs.

(3) In Erasing Operation,

The voltage first applied to the well which may be formed in the upperportion of the semiconductor substrate and above which the memory cellmay be arranged may be set within the range of, for example, 12 V to13.6 V. However, the voltage is not limited to this and may be setwithin the range of, for example, 13.6 V to 14.8 V, 14.8 V to 19.0 V,19.0 V to 19.8 V, or 19.8 V to 21 V.

A time (tErase) of the erasing operation may be set within the range of,for example, 3,000 μs to 4,000 μs, 4,000 μs to 5,000 μs, or 4,000 μs to9,000 μs.

(4) The Structure of the Memory Cell

A charge accumulation layer may be arranged on a 4 to 10 nm thick tunnelinsulating film. The charge accumulation layer may have a stackedstructure of a 2 to 3 nm thick insulating film of SiN or SiON and 3 to 8nm thick polysilicon. A metal such as Ru may be added to thepolysilicon. An insulating film is provided on the charge accumulationlayer. The insulating film may include a 4 to 10 nm thick silicon oxidefilm sandwiched between a 3 to 10 nm thick lower High-k film and a 3 to10 nm thick upper High-k film. As the High-k film, HfO or the like maybe usable. The silicon oxide film may be thicker than the High-k film. A30 to 70 nm thick control electrode may be formed on a 3 to 10 nm thickwork function adjusting material on the insulating film. Here, the workfunction adjusting material may be a metal oxide film such as TaO or ametal nitride film such as TaN. As the control electrode, W or the likeis usable.

An air gap may be formed between the memory cells.

In the above embodiments, a NAND flash memory has been exemplified asthe semiconductor storage device. However, the embodiments may beapplicable not only to the NAND flash memory but also to other generalsemiconductor memories, and also applicable to various kinds of storagedevices other than the semiconductor memories. In the flowchartsdescribed in the above embodiments, the order of processes may bechanged as long as it is possible.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A memory system, wherein the memory systemcomprises: a memory controller; a plurality of memory cells; and aplurality of word lines, wherein a first word line of the plurality ofword lines is coupled to a first memory cell of the plurality of memorycells; a second word line of the plurality of word lines is coupled to asecond memory cell of the plurality of memory cells; a third word lineof the plurality of word lines is coupled to a third memory cell of theplurality of memory cells; the memory controller is configured to: senda first command in response to receiving a read command from a host; andsend a second command in response to determining a shift voltage of thefirst command exceeds a first threshold and does not exceed a secondthreshold; and the memory system is configured to: apply a first set ofvoltages to the first word line, the second word line, and the thirdword line in response to the first command; and apply a second set ofvoltages, different from the first set of voltages, to the first wordline, the second word line, and the third word line in response to thesecond command.
 2. The memory system of claim 1, wherein the first wordline is adjacent to the second word line; and the second word line isadjacent to the third word line.
 3. The memory system of claim 1,wherein no data is read from the plurality of memory cells in responseto the second command.
 4. The memory system of claim 1, wherein data isread from a dummy word line of toe plurality of word lines in responseto the second command.
 5. The memory system of claim 1, wherein thefirst set of voltages comprises: a first voltage for the first word lineand the third word line; a second voltage for the second word line; anda third voltage for a remaining set of word lines of the plurality ofword lines, wherein the first voltage is higher than the third voltage.6. The memory system of claim 1, wherein the second set of voltages is asame voltage for each word line of the plurality of word lines.
 7. Thememory system of claim 1, wherein the first threshold is a minimumacceptable index of a shift voltage table and the second threshold is amaximum acceptable index of the shift voltage table.
 8. The memorysystem of claim 7, wherein the minimum acceptable index of the shiftvoltage table is associated with a first shift voltage; the maximumacceptable index of the shift voltage table is associated with a secondshift voltage; and the second shift voltage is higher than the firstshift voltage.
 9. The memory system of claim 1, wherein the memorycontroller is further configured to send a plurality of second commandsuntil a number of second commands performed on a logical blockassociated with the plurality of word lines exceeds a predeterminedthreshold.
 10. The memory system of claim 1, the memory controller isfurther configured to reduce a shift voltage value associated with theshift voltage in response to sending the second command.
 11. A method ofmanaging a memory system, wherein the method comprises: sending a firstcommand in response to receiving a read command from a host; sending asecond command in response to determining a shift voltage of the firstcommand exceeds a first threshold and does not exceed a secondthreshold; applying a first set of voltages to a first word line, asecond word line, and a third word line of a plurality of word lines inresponse to the first command; and applying a second set of voltages,different from the first set of voltages, to the first word line, thesecond word line, and the third word line in response to the secondcommand.
 12. The method of claim 11, wherein the first word line isadjacent to the second word line; and the second word line is adjacentto the third word line.
 13. The method of claim 11, wherein the methodfurther comprises transmitting data to the host in response to the firstcommand and not transmitting data to the host in response to the secondcommand.
 14. The method of claim 11, wherein the method furthercomprises reading data from a dummy word line of the plurality of wordlines in response to the second command.
 15. The method of claim 11,wherein applying the first set of voltages comprises: applying a firstvoltage to the first word line and the third word line; applying asecond voltage to the second word line; and applying a third voltage toa remaining set of word lines of the plurality of word lines, whereinthe first voltage is higher than the third voltage.
 16. The method ofclaim 11, wherein applying the second set of voltages comprises applyinga same voltage to each word line of the plurality of word lines.
 17. Themethod of claim 11, wherein the first threshold is a minimum acceptableindex of a shift voltage table and the second threshold is a maximumacceptable index of the shift voltage table.
 18. The method of claim 17,wherein the minimum acceptable index of the shift voltage table isassociated with a first shift voltage; the maximum acceptable index ofthe shift voltage table is associated with a second shift voltage; andthe second shift voltage is higher than the first shift voltage.
 19. Themethod of claim 11, wherein the method further comprises: sending aplurality of second commands until a number of second commands performedon a logical block associated with the plurality of word lines exceeds apredetermined threshold.
 20. The method of claim 11, wherein the methodfurther comprises: reducing a shift voltage value associated with theshift voltage in response to sending the second command.